1. Field of the Invention
The present invention relates to a digital signal transmission suitable for a digital broadcasting to mobile receivers, and more particularly, to a digital signal transmission using an OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme.
2. Description of the Background Art
A digital signal transmission using an OFDM modulation scheme (referred hereafter as the OFDM digital signal transmission scheme) in which the digital signals are transmitted by using a number of mutually orthogonal carriers is known to have various advantages such as its capability to deal with multi-paths, its relatively high frequency utilization efficiency, and its nearly Gaussian white noise like spectrum which causes very little interference with other communication services. For these reasons, the OFDM modulation scheme is considered to be particularly suitable for the application such as a digital audio broadcasting to mobile receivers.
However, in comparison with a conventionally known digital modulation scheme using a single carrier, it is rather difficult in this OFDM modulation scheme to make a coherent demodulation by recovering carriers at a receiver side. For this reason, in order to apply the OFDM digital signal transmission scheme to a conventionally developed digital sound broadcasting in which each carrier is to be QPSK (Quadrature Phase Shift Keying) modulated, a signal transmission must be realized by using differential modulation and demodulation.
On the other hand, in recent years, the OFDM modulation scheme has also been attracting attentions in view of its application to the digital TV signal transmission. In such an application, it is preferable to use a multi-level modulation such as 16 QAM (Quadrature Amplitude Modulation) to each carrier so as to increase the transmission bit rate.
However, in a case of using the multi-level modulation such as 16 QAM in which the data are carried not only by the phases of the signals but also by the amplitudes of the signals, it is difficult to reproduce the transmitted data by using the differential demodulation without making a recovery of carriers at a receiver side.
In order to cope with this problem, it is possible to consider a scheme in which a transmitter side provides reference data for the amplitude and the phase of each carrier at predetermined positions within each transmission signal frame such that a receiver side can reproduce valid transmission data according to the reference data transmitted by the transmission signals.
However, this type of scheme is associated with the following problems.
Namely, in a case the reference data for all the carriers are transmitted by one symbol, if the average power of the reference data is larger than the average power of the valid transmission data, the average power of the symbol for transmitting the reference data will become undesirably larger than the average power of the other symbols for transmitting the valid transmission data.
In addition, in such a case in which the reference data for all the carriers are transmitted by only one symbol, if the symbol for transmitting the reference data has an error, a very long burst error will be caused.
Also, in a case only one value is used for the reference data, the signal peak level will be large such that the signal transmission will be easily influenced by the non-linearity of the transmission path.
Consequently, there has been no conventionally known OFDM digital signal transmission scheme in which the amplitude and phase reference data can be interleaved in the transmission signals, and the valid transmission data can be demodulated by using the reference data without requiring a recovery of the carriers at a receiver side.